The present invention relates to apparatus for fabricating large generally cylindrical structures from one or more metal sheets, and more particularly, to an apparatus for superplastic forming of such structures having substantial deformations.
For many years it has been known that certain metals, such as Titanium, as well as certain alloys, exhibit superplasticity within limited temperature ranges and strain rates. Superplasticity is the capability of a material to develop unusually high tensile elongations with a reduced tendency towards necking. Thus when in a superplastic condition, the metal or metal alloy exhibits low resistance to deformation and may be elongated with controlled thinning. This permits a sheet of such metal to be rapidly formed against dies to achieve desired shapes. Superplastic forming (SPF) may be performed in conjunction with diffusion bonding (DB). Diffusion bonding refers to a metallurgical joining of surfaces of similar or dissimilar metals by holding them in physical contact and applying heat and pressure sufficient to cause commingling of the atoms at the junction. See for example U.S. Pat. Nos. 3,934,441 of Hamilton et al.; 3,927,817 of Hamilton et al.; 4,984,348 of Cadwell; and 5,016,805 of Cadwell.
In the aerospace industry, it is frequently necessary to form large generally cylindrical structures. As used herein, the term "generally cylindrical" refers to surfaces of revolution such as those obtained by rotating a line or curve about a central axis. These include cylinders, cones, frusto-conical structures and tubular configurations with undulating sidewalls. An example of such a structure is an exhaust mixer of a turbofan aircraft engine. Typically it comprises a plurality of axially extending, circumferentially spaced lobes which surround the core engine. The lobes mix the fan air and the hot exhaust gas from the turbine to obtain improved noise suppression and/or engine performance. Representative configurations of such exhaust mixers are disclosed in U.S. Pat. Nos. 4,077,206 of Ayyagari and 4,149,375 of Wynosky et al.
Large generally cylindrical structures utilized in connection with aircraft are frequently made of Titanium or other metal alloy utilizing SPF. U.S. Pat. No. 4,429,824 of Woodward discloses a method of combining SPF and DB to form a cylindrical shaped part from a pair of Titanium sheets which are first rolled into cylinders and concentrically juxtaposed. The tooling includes an internal mandrel made of a material having a relatively high coefficient of thermal expansion and an external mandrel made of a material having a relatively low coefficient of thermal expansion. These mandrels are referred to as "Delta-Alpha" tooling. Forming shims are positioned within the tooling to predesignate areas of diffusion bonding when the applied pressure and differential expansion of the tooling forces the sheets of material together at the shim locations.
Because of the limited amount of differential thermal expansion in the Delta-Alpha tooling, it is not suitable for forming large cylindrical structures by SPF when substantial deformations in the radial direction are required. It has been known, therefore, to roll a sheet of Titanium into a cylinder, and then to weld circular shaped sheets of Titanium to the upper and lower ends of the cylinder to form a gas tight container. Argon gas can then be introduced into the sealed container, after it has been heated to a temperature at which it exhibits superplasticity, in order to blow the cylindrical sheet radially outwardly against ceramic dies. The problem with this approach is that it is time consuming to weld the circular Titanium sheets to the upper and lower ends of the cylindrical Titanium sheet. When the formed part has cooled, it is necessary to cut off the ends of the part. The amount of scrap Titanium generated is substantial. This Titanium must be completely reprocessed before it can results in significant cost in fabricating such parts. be used again in the fabrication of aircraft parts. This
My U.S. Pat. No. 4,984,732 discloses a method of superplastically forming and diffusion bonding a generally cylindrical laminate structure in order to provide for a minimum thicknesses at certain points of extreme deformation. In the method disclosed in my aforementioned U.S. Pat. No. 4,984,732, a sealing ring is positioned between the upper edge of an outer Titanium cylinder and the upper edge of a seal diaphragm. Another sealing ring adjacent a support holds the seal diaphragm tightly against the sealing ring. In this method, the cylinders must be positioned over an inner mandrel made, for example, of corrosion resistant stainless steel.
Despite the improvements disclosed in my aforementioned U.S. Pat. No. 4,984,732, the sealing of large diameter generally cylindrical structures at high temperatures, e.g. 1,700.degree. F. or more, so that they can be pressurized for SPF/DB operations has been a major and costly problem.
It would be desirable to provide an improved SPF apparatus for fabricating large generally cylindrical structures without the need for the inner mandrel and complex seal arrangement disclosed in my aforementioned U.S. Pat. No. 4,984,732.